1. Field of the Invention
The present invention relates to optical communication equipment.
2. Description of the Related Art
Optical communication systems employ optical amplifiers, e.g., to compensate for signal attenuation in optical fibers. One type of amplifier that may be used in a fiber-based optical communication system is an optical parametric amplifier (OPA). As known in the art, an OPA is a device that produces a tunable coherent optical output via nonlinear optical processes, in which, typically, one or two pump-wave photons are converted into two new photons with conservation of photon energy and momentum. The waves corresponding to the two new photons are usually referred to as a signal wave and an idler wave, respectively. In an OPA, noise levels at the input and the output are comparable and the idler wave is a phase conjugate of the signal wave.
FIG. 1 shows a representative OPA 100 of the prior art that is configured for use in a long-haul transmission line of an optical communication system. OPA 100 is coupled between two sections 102 and 102′ of a long-haul optical fiber. OPA 100 has a coupler 104 configured to combine an optical communication signal from section 102 with a pump wave generated by a pump-wave source 106 (e.g., a laser). Depending on the implementation of OPA 100, the pump wave may be a continuous-wave (CW) or pulsed optical signal. The combined signal is directed into a highly nonlinear fiber (HNLF) 108, where the optical communication signal is amplified by way of parametric amplification. A filter 110 placed at the end of HNLF 108 separates the amplified optical communication signal (e.g., from the pump wave and an idler signal generated in HNLF 108) for further transmission in the communication system via section 102′.
One attractive feature of OPA 100 is that it can be designed to provide signal amplification at arbitrary wavelengths. In addition, OPA 100 can be configured to conjugate signals and/or change their wavelengths. However, one problem with OPA 100 is that the spectral width of its gain band may be relatively narrow. Also, the spectral shape of that band is typically not flat. One additional problem is that the intensity of the pump wave and therefore the gain in OPA 100 are limited by Brillouin scattering. These problems impede the use of OPAs in optical communication systems.